Rapid advances in biotechnology have led to the discovery of numerous protein and peptide therapeutics, many of which have recently reached the marketplace or are currently under regulatory review by the United States Food and Drug Administration. Unlike traditional small-molecule drugs, however, proteins and peptides generally cannot be administered orally; injection or infusion is most often required. Further, because of their fragility and short in vivo half-lives, encapsulation of proteins in biodegradable polymeric devices, from which the drug can be delivered, locally or systemically, for a prolonged period of time, has been a promising and intensely studied solution to these problems. Biodegradable microspheres comprising a variety of polymers have been the most studied devices due to relatively simple fabrication and facile administration to a variety of locations in vivo through a syringe needle.
Several methodologies for microsphere fabrication have been described, including precipitation, spraying, phase separation, and emulsion techniques. The emulsion and spraying approaches have been commonly used both at the bench and industrial scales. Sphere size and size distribution are reproducible but often poorly controllable. Standard deviations equal to 25-50% of the mean diameter are not uncommon.
Control of sphere size and size distribution has several important implications for controlled-release drug delivery. For example, there typically is an ideal sphere size that provides a desired release rate and route of administration. Spheres that are xe2x80x9ctoo smallxe2x80x9d exhibit poor encapsulation efficiency, may migrate from the site of injection, and may exhibit undesirably rapid release of their payload. Spheres that are xe2x80x9ctoo largexe2x80x9d may not easily pass through a syringe needle. Thus, the typically polydisperse spheres generated by conventional fabrication techniques must be filtered or sieved to isolate particles within the desired size range, and the polymer and drug composing spheres outside that range are wasted.
Uniform microspheres approximately 1-5 xcexcm in diameter would be ideal for passive targeting of professional antigen-presenting cells (APCs) such as macrophages and dendritic cells. Similarly, microspheres 10-20 xcexcm in diameter could be used to target the tortuous capillary bed of tumor tissues by chemoembolization. A system capable of precise microsphere fabrication could allow the optimal size for such applications to be identified and provide an efficient route to commercial manufacture and clinical implementation.
A long-sought goal for controlled-release drug delivery technologies is the ability to precisely control the release rate of encapsulated compounds, and microsphere size is a major determinant of release kinetics. Larger spheres generally release encapsulated compounds more slowly and over longer time periods, other properties (polymer molecular weight, initial porosity, drug distribution within the sphere, etc.) being equal. A constant (i.e., zero-order) release rate is often preferred, while variable drug release rates can be beneficial for many important indications. For example, intermittent high doses of antibiotics may alleviate evolution of resistance in bacteria, and discontinuous administration of vaccines often enhances the immune response.
Methods to control drug release rate include (i) choice of polymer chemistry (anhydrides, esters, etc.) and comonomer ratios, (ii) conjugating the drug to the polymer, (iii) varying the microsphere formulation parameters, and thus the physical characteristics of the resulting particles, and (iv) manipulating the sphere size and distribution. The success of the latter studies was limited by the relatively broad microsphere size distributions.
In recent years, there have been several reports of the fabrication of biodegradable polymer microspheres with controlled, uniform size (P. Sansdrap and A. J. Moes, Influence of manufacturing parameters on the size characteristics and the release profiles of nifedipine from poly(-zDL-lactide-co-glycolide) microspheres. Int. J. Pharm. 98 (1993) 157-164; B. G. Amsden and M. Goosen, An examination of the factors affecting the size, distribution, and release characteristics of polymer microbeads made using electrostatics. J. Control. Release 43 (1997) 183-196; K. Shiga, N. Muramatsu and T. Kondo, Preparation of poly(D,L-lactide) and copoly(lactide-glycolide) microspheres of uniform size. J. Pharm. Pharmacol. 48 (1996) 891-895; B. Amsden, The production of uniformly sized polymer microspheres. Pharm. Res. 16 (1999) 1140-1143; and N. Leelarasamee, S. A. Howard, C. J. Malanga and J. K. H. Ma, A method for the preparation of polylactic acid microcapsules of controlled particle size and drug loading. J. Microencapsul. 5 (1988) 147-157). However, none of these methods was successful in generating particles in a size range appropriate for drug delivery (xcx9c1-100 xcexcm) while maintaining narrow size distributions. In addition, these previous methods appear to be difficult to scale-up for commercial applications.
Hollow sphere fabrication techniques are disclosed in N. K. Kim, K. Kim, D. A. Payne, and R. S. Upadhye, xe2x80x9cFabrication of hollow silica aerogel spheres by a droplet generation method and sol-gel processing,xe2x80x9d J. Vac. Sci., Technol. A., vol. 7, no.3 pp. 1181-1184 (1989) and K. Kim, K. Y. Jang and R. S. Upadhye, xe2x80x9cHollow silica spheres of controlled size and porosity by sol-gel processing,xe2x80x9d J. Am. Ceram. Soc., 74:8, pp. 1987-1992, (1991).
Electrostatic spraying technique is disclosed in K. Kim and R. J. Turnbull, xe2x80x9cGeneration of charged drops of insulating liquids by electrostatic spraying,xe2x80x9d J. Appl. Phys., vol. 47, no. 5, pp. 1964-1969, May 1976, U.S. Pat. No. 5,344,676 to Kim et al., and U.S. Pat. No. 6,060,128 to Kim, et al.
Previously developed techniques designed to fabricate hollow spheres employ a dual-nozzle scheme in which two coaxially mounted nozzles carrying different materials in liquid phase (the material in the inner nozzle could also be a gas) produce a smooth cylindrical jet which, in turn, is broken up into uniform droplets by an acoustic excitation. (See N. K. Kim, et al., xe2x80x9cFabrication of hollow silica aerogel spheres by a droplet generation method and sol-gel processing,xe2x80x9d infra and K. Kim et al., xe2x80x9cHollow silica spheres of controlled size and porosity by sol-gel processing,xe2x80x9d infra). The smallest drops that can be made with this method are roughly twice as large as the opening of the outer nozzle. This in turn indicates practical difficulties associated with fabricating uniform solid and hollow spheres of small sizes (less than about 50 xcexcm in diameter) especially spheres in the submicron-size range. The reason is that the smaller the nozzle opening, the greater the chances for it to get plugged up, especially if the pharmaceutical compounds to be encapsulated are suspended as a particulate in the sphere-forming liquid. This problem becomes worse when the materials being used are viscous.
With previous technologies for spraying microdroplets from nozzle-type devices, the minimum sphere size typically obtainable is limited by the size of the nozzle opening. Usually, it is not possible to make drops smaller than the nozzle opening; typically, droplet diameters are 1-4 times the diameter of the nozzle. This presents several difficulties as the desired sphere size decreases. One problem is that fabrication of the nozzles themselves becomes more difficult as size decreases. This is especially true for large-scale fabrication methods in which it is necessary to form droplets through arrays of nozzles (perhaps 1000-2000). A second limitation stems from the pressure needed to pump fluids through small nozzles. The pressure required is given by       Δ    ⁢          xe2x80x83        ⁢    p    =            8      ⁢      μ      ⁢              xe2x80x83            ⁢      LQ              π      ⁢              xe2x80x83            ⁢              R        4            
where xcex94p is the pressure drop across the nozzle, xcexc is the viscosity of the fluid, L is the length of the nozzle xe2x80x9cpassagexe2x80x9d, Q is the volumetric flow rate of the fluid passing through the nozzle, and R is the radius of the nozzle opening. Thus, the pressure required scales with Rxe2x88x924. If one wishes to make microdroplets of xcx9c5 xcexcm diameter, traditional methods may require a nozzle with a diameter of 5 xcexcm or less. For example, at a flow rate of 1 mL/min and a fluid viscosity of 100 centipoise (100-times more viscous than water), a 5-xcexcm diameter orifice would require a pump head of xcx9c1.1xc3x971010 Pa (xcx9c110,000 atm). This is clearly an impossibly high pressure. Even water, xcexcxcx9c1 cp, requires a pressure of 1,100 atm to be pumped through a 5-xcexcm diameter nozzle at 1 mL/min. Thus, pumping virtually any liquid through a nozzle of 5-xcexcm diameter would require special equipment, if it could be done at all.
Another problem with traditional methods of forming small spheres is that some compounds to be encapsulated, such as plasmid DNA, may be damaged by shear forces. Damage depends on the product of the shear rate, xcex3, and the time spent in the shear field, xcex8. The average value of this product for a fluid flowing through a pipe is given by
(xcex3xcex8)avg=16/3xc2x7(L/D)
where L is the length of the pipe and D is the pipe diameter. The orifice of a nozzle can be approximated as a pipe. However, entrance effects will tend to increase the shear rate meaning this equation will give a low estimate. Regardless, the value of xcex3xcex8 is approximately inversely proportional to the diameter of the orifice. Thus, decreasing the nozzle diameter from 100 to 5 xcexcm would increase the damage done to any encapsulated compound by a factor of 20.
U.S. Pat. No. 6,116,516 describes stabilized capillary microjets, that produce aerosols. The microjets are formed by forcing a gas around a liquid stream. Under the correct conditions, micron-sized aerosols are produced, where preferably 90% or more have the same diameter plus/minus 3% to 30%.
It is an object of this invention to produce micro- and nano-sized spherical particles by pumping material through a small orifice and then shaking said liquid with an acoustic type-wave.
It is also an object of this invention to produce micro- and nano-sized spherical particles by pumping a material through a small orifice and adding an additional downward force, said downward force comprising either electrohydrodynamic technique or a second liquid stream adjacent and parallel to the liquid at a velocity greater than the first liquid. It is also an object of this invention to produce uniform micro- a nd nano-sized spherical particles by using acoustic type waves with the above process.
It is a further object of this invention to produce hollow micro- a nd nano-sized spherical particles by utilizing an inside and outside liquid that are passed through one or the other of two coaxially mounted nozzles to produce a smooth cylindrical jet of the outside liquid coaxially containing the inside liquid (or gas). This jet can be further broken into uniform droplets by acoustic waves.
It is yet a further object of this invention to provide a novel process for hardening micro- and nano-spheres by utilizing any of the above processes for producing micro- and nano-spheres, wherein the nozzle or orifice utilized is placed beneath the surface of an aqueous bath, to allow hardening of the spheres with a minimum of deformation.
It is still a further object of this invention to produce therapeutic compounds, encapsulated by any of the above techniques useful as biomedical compositions for medical treatment of humans and therapeutic value.
It is yet another object of this invention to produce multi-shelled micro- and nano-spheres of controlled sizes, shell thicknesses and number of shells with different shells comprising different materials useful for biomedical applications for humans and animals including controlled-release drug delivery systems.
It is still a further object of this invention to produce micro- a nd nano-spheres of the types described above, and other types, for biomedical applications including passive or active targeting to desired cells, tissues or regions of the body.
It is a further object of this invention to provide an apparatus and process for producing micro- and nano-spherical particles of precisely controlled sizes and size distributions for biomedical applications, especially controlled-release drug delivery systems.
It is a further object of this invention to produce stable micro- and nano-spheres of a desired size, chemical composition and stoichiometry.
It is a further object of this invention to produce micro- and nano-spheres of controlled sizes for biomedical applications including controlled-release drug delivery systems.
It is yet a further object of this invention to produce hollow micro- and nano-spheres of controlled sizes for biomedical applications including controlled-release drug delivery systems.
These and other objects are provided in this invention which is described in more detail hereafter.
In a first aspect, the present invention is a method of forming particles, comprising accelerating a first stream comprising a first liquid; and vibrating the first stream, to form particles.
In a second aspect, the present invention is a method of forming particles, comprising accelerating a first stream comprising a first liquid. The accelerating comprises applying charge to the first stream. The particles comprise a core and a shell.
In a third aspect, the present invention is particles having an average diameter of 50 to 100 xcexcm. Ninety percent of the particles have a diameter that is within 2% of an average diameter of the particles.
In a fourth aspect, the present invention is particles having an average diameter of 1 to 50 xcexcm. Ninety percent of the particles have a diameter that is within 1 xcexcm of an average diameter of the particles.
In a fifth aspect, the present invention is particles, prepared by the above method.
In a sixth aspect, the present invention is an apparatus for forming particles, comprising (i) a first nozzle, for forming a first stream of a first liquid, (ii) a second nozzle, oriented for forming a second stream of a second liquid in contact with the first stream, and (iii) a vibrator, for forming particles from the first stream.
In a seventh aspect, the present invention is an apparatus for forming particles, comprising (i) a first nozzle, for forming a first stream of a first liquid, (ii) a charge source, for applying charge to the first stream, and (iii) a vibrator, for forming particles from the first stream.
In an eighth aspect, the present invention is an apparatus for forming particles, comprising (i) means for forming a first stream of a first liquid, (ii) means for accelerating the first stream, and (iii) means for vibrating the first stream.
In a ninth aspect, the present invention is an apparatus for forming particles, comprising (i) a first nozzle, for forming a first stream of a first liquid, (ii) a second nozzle surrounding the first nozzle, for forming a second stream of a second liquid surrounding the first stream, (iii) a charge source, for applying charge to at least one of the first and second streams.
In a tenth aspect, the present invention is a method of making particles, comprising forming particles with the above apparatuses.